The use of high frequency ultrasonic waves for obtaining information from objects is well known. For example, high frequency ultrasonic waves are often used in the medical field to obtain information relating to internal elements, organs and the structure of living bodies.
Ultrasonic wave transmission is particularly useful in determining the distance between various elements and organs in living bodies, and especially in determining distances between various elements of the human eye.
One of the most useful applications of ultrasonic wave transmission is a measure of the axial length of the eye, i.e., the measure of the distance from the cornea to the retinal surface. This distance allows an ophthamologist to compute a refractive power of the eye be well known methods and then to specify the correct intraocular lens for patients undergoing, for example, cataract surgery. In fact, although optical methods for measuring this axial distance are known, the ultrasonic wave transmission method is the method of choice for making axial length measurements in patients whose cataracts prevent sufficient transmission of light to the retina.
The technology and technique for making measurements in ophthalmology, including determination of the axial length of the human eye by utilizing ultrasonic wave transmission is disclosed in Coleman, D., Ultrasonography of the Eye and Orbit, Lea & Febiger (1977). In an illustrative method of making such measurement, referred to in the art as "Ultrasonic A-Scan", a hand held probe-like device is placed in contact with a cornea of the human eye. The probe-like device comprises a transducer assembly and a separate fixation light which the patient is instructed to view. As the patient views the fixation light, high frequency ultrasonic waves, preferably of very short duration, are transmitted by a transmitter of the transducer assembly into the eye. These high frequency ultrasonic waves are reflected from various anatomical structures within the eye. More specifically, a first reflection occurs at an anterior corneal surface of the eye and a last reflection occurs at a retinal surface of the eye. Intermediate reflections include a second reflection at a posterior corneal surface, a third reflection at an anterior lens surface and a fourth reflection at a posterior lens surface.
These five reflected ultrasonic waves, or echoes, are sensed by an ultrasonic transducer assembly in the probe and converted to electrical impulses which are displayed on a display device. If the velocity at which the waves travel through the anatomical structures of the eye is known, the difference in time taken for two successive echoes from different structures within the eye to reach the transducer is a measure of the physical distance between these structures. The sound velocity of the eye and its anatomical structures has been very carefully measured and are well known.
A description of a known transducer assembly may be found in U.S. Pat. No. 4,213,464 to Katz et al., while a description of a known biometric measuring device may be found in U.S. Pat. No. 4,154,114 to Katz et al. Each of these two U.S. patents are expressly incorporated herein by reference. U.S. Pat. Nos. 4,508,121 to Myers, 4,564,018 to Hutchison et al. and 4,576,176 to Myers further disclose ocular measurement devices and are also incorporated herein by reference.
The measured physical distances between various structures of the eye is accurate only to the extent that the eye is not deformed when such measurements are made. Should the eye structure be deformed during the measurement process, any resulting measured distances will not be representative of actual distances and will lead to replacement lenses of improper refractive power, thereby depriving a patient of full use of his vision.
Unfortunately, known devices in attempting to measure distances between various anatomical structures of the eye introduce a deforming force to the eye and thereby result in inaccurate measurements. For example, in measuring an axial length of the eye, known ultrasonic transducers are placed upon and forcefully held in contact with the anterior surface of the cornea. In use of such known transducers, especially transducers of the hand held variety, any hand movement will result in a change of pressure applied to the eye by the transducer assembly of the probe-like device. Should such pressure exceed the pressure exerted by the fluids within the eye and the eye structure itself, the corneal surface may be deformed. Under typical conditions the pressure of the fluids within the eye has been found to be approximately 14 mm Hg.
Measurements of axial lengths taken under these conditions using the ultrasonic wave transmission method will produce an error since the cornea and possibly other structures of the eye are inadvertently slightly flattened due to pressure applied by the transducer assembly. The flattening is called applanation and represents the chief source of error in ultrasonic A-Scan measurements.
Since the ultrasonic probe-like device is a rigid structure, is typically hand held and the pressures within the eye are small compared to those that the human hand can produce in even a soft application of the probe to the eye, it is extremely difficult to remove applanation errors in ultrasonic A-Scan measurements. If a non-distorting contact between the probe and the cornea could be ensured, it is believed that axial length measurements could be made to within 0.01 mm. This would represent an increase of an entire order of magnitude in accuracy over properly used ultrasonic A-Scan systems which are capable of measuring distances to within 0.1 mm.